Focused ultrasound (i.e., acoustic waves having a frequency greater than about 20 kiloHertz) can be used to image or therapeutically treat internal body tissues within a patient. For example, ultrasound waves may be used in applications involving ablation of tumors, thereby eliminating the need for invasive surgery, targeted drug delivery, control of the blood-brain barrier, lysing of clots, and other surgical procedures. During tumor ablation, a piezoceramic transducer is placed externally to the patient, but in close proximity to the tissue to be ablated (i.e., the target). The transducer converts an electronic drive signal into mechanical vibrations, resulting in the emission of acoustic waves. The transducer may be geometrically shaped and positioned along with other such transducers so that the ultrasound energy they emit collectively forms a focused beam at a “focal zone” corresponding to (or within) the target tissue region. Alternatively or additionally, a single transducer may be formed of a plurality of individually driven transducer elements whose phases can each be controlled independently. Such a “phased-array” transducer facilitates steering the focal zone to different locations by adjusting the relative phases among the transducers. As used herein, the term “element” means either an individual transducer in an array or an independently drivable portion of a single transducer. Magnetic resonance imaging (MRI) may be used to visualize the patient and target, and thereby to guide the ultrasound beam.
The noninvasive nature of ultrasound surgery is particularly appealing for the treatment of brain tumors. Moreover, coherent, non-invasive focusing of ultrasound through the human skull has been considered as a tool for targeted drug delivery to the brain, improved thrombolytic stroke treatment, blood flow imaging, the detection of internal bleeding, and tomographic brain imaging. However, the human skull has been a barrier to the clinical realization of ultrasound therapy. Impediments to transcranial ultrasound procedures include strong attenuation and the distortions caused by irregularities in the skull's shape, density, and sound speed, which contribute toward destroying the focus and/or decreasing the ability to spatially register received diagnostic information.
In addition, during a focused ultrasound procedure, small gas bubbles (or “microbubbles”) may be generated in the liquid contained in the brain tissue, e.g., due to the stress resulting from negative pressure produced by the propagating ultrasonic waves and/or from when the heated liquid ruptures and is filled with gas/vapor. The reaction of tissue containing a higher relative percentage of microbubbles during the continued application of the ultrasound energy is non-linear and difficult to predict. For example, microbubbles may reflect and/or scatter ultrasound waves, and further deteriorate the focus or reduce the intensity thereof. Additionally, the microbubbles may collapse due to the applied stress from an acoustic field; this mechanism, called “cavitation,” may cause extensive tissue damage beyond that targeted, and may be difficult to control. Finally, because microbubbles are typically generated and/or spread in the patient's body in a non-uniform manner, microbubbles accumulating in the skull may further increase the challenge of accounting for the ultrasound distortions resulting from both the skull and microbubbles when estimating/calculating focusing properties.
Accordingly, there is a need to minimize microbubble interference with therapeutic ultrasound waves in order to optimize focusing properties and maximize the amount of acoustic energy available at the focus.